The “EmDrive” claims to make the impossible possible: a method of pushing spacecraft around without the need for — well, pushing. No propulsion. No exhaust. Just plug it in, fire it up and you can cruise to the destination of your dreams.
But the EmDrive doesn’t just violate our fundamental understanding of the universe; the experiments that claim to measure an effect haven’t been replicated. When it comes to the EmDrive, keep dreaming.
It goes by various names — the EmDrive, the Q-Drive, the RF Resonant Cavity, the Impossible Drive — but all the incarnations of the device claim to do the same thing: bounce some radiation around inside a closed chamber, and presto-chango you can get propulsion.
This is a big deal, because all forms of rocketry (and indeed, all forms of motion across the entire universe) require conservation of momentum. In order to set yourself in motion, you have to push off of something. Your feet push off of the ground, airplanes push themselves off of the air, and rockets push parts of themselves (e.g., an exhaust gas) out the back end to make them go forward.
But the EmDrive doesn’t. It’s just a box with microwaves inside it, bouncing around. And supposedly it is able to move itself.